This invention relates to a process for preparing cyclobutenoarenes. In a specific aspect, the invention relates to a pyrolytic process for the production of benzocyclobutene.
The four-membered ring of benzocyclobutenes is known to open at elevated temperature to form a very reactive diene which rapidly dimerizes and polymerizes. Molecules containing two or more benzocyclobutene groups are therefore useful as heat-curable thermosetting resins. Also, elastomers or thermoplastics containing benzocyclobutene substitutents crosslink on heating. In order to economically prepare benzocyclobutene-functional resins or polymers, however, an economic method of generating the benzocyclobutene structure itself must be found.
In the past, the benzocyclobutene structure has been synthesized most commonly by the pyrolysis of o-methylbenzyl halides (which may or may not contain additional substituents) at temperatures above 650.degree. C. and pressures below 1 mm Hg. Optionally, an inert diluent is added to enable operation at higher pressures (in the 25 to 200 mm Hg range). With or without an inert diluent, however, the major problem in the reaction is caused by by-product HCl (or other hydrogen halide) formation.
HCl gas tends to corrode the steel used in vacuum pumps. Even if special corrosion-resistant vacuum pumps are used in maintaining the low reaction pressures for flash vacuum pyrolysis, the HCl tends to be absorbed by and to degrade the pump oil, requiring frequent oil changes. The standard way of protecting vacuum pumps from HCl is to pass the gas stream through a packed bed of alkaline material (such as soda lime) upstream from the pump. However, these solid alkaline materials are limited in their ability to absorb acid gases. The alkalies tend to react at the surface with the HCl or other acid gases, with particles of alkali tending to become coated with a layer of halides so that the interior of each particle of alkali remains unreacted. Hence alkali beds must be changed very frequently if used to absorb acid gases formed in stoichiometric amounts in a reaction.
It is therefore an object of the present invention to provide a process for the preparation of cyclobutenoarenes. In one embodiment, it is an object of the invention to provide a relatively non-corrosive pyrolytic process for the preparation of benzocyclobutenes.